Having a valid clock signal will quickly lead to validating the 6560 VIC chip and 6502 CPU; but for that to happen we will quickly require a valid set of ROM chips. Are the ROM on our VIC-20s valid?
Similarly to the Commodore 64, the VIC-20 has 3 ROM:
- the Character ROM to display something legible by humans – in theory somewhat similar between the two systems;
- the BASIC ROM to let humans program in BASIC, but also serves as the main command line interface of the computer, and those are tied to the machine’s Kernal;
- and the Kernal ROM – yes, kernal with an A – that’s how Commodore named it.
While these are generally reliable, they can fail. More specifically, one of our PCB’s ROM chips came with masking tape that has a big X and a question mark on it, making it very doubtful that it could be a usable chip. Are others also meant to have such a masking tape question and X mark?
An easy way to test would be to put them in a working VIC-20 and see what’s going on… but without an available working VIC-20, something that can be done is dumping them to file in order to read the content.
2332 masked ROM
Similarly to the C64, the character ROM is a 2332 masked ROM chip; we can therefore use the same adapter as with the 2532 we built for programming Commodore 64 character ROMs.
With the adapter, it reads as an 2732 EPROM. The content of both character ROMs seems to look good.
2364 masked ROM
The Kernal and BASIC ROMs are 2364 masked ROM, We recently made a 2364-2764 adapter for the specific purpose of dumping these ROM chips.
Of 6 chips, 5 seems to read good, with only the X-marked Kernal ROM from VIC20#1 being weird – which isn’t really a surprise.
Binary comparison (we like beyond compare – no affiliations) of the Character ROMs shows they are both identical.
The BASIC ROMs have a total of 2 different bytes (perhaps even bits). Being that Zimmers.net stated there is only 1 version of the BASIC ROM, one of the chip may have slight issues, and maybe cause problems? It may be on the verge of becoming a bigger problem, or could work as-is for years. Who knows.
One Kernal ROM being already declared as bad makes that we have no comparison to make between the two, but we can download rom dumps from the interwebs…
Binary comparing the files with the binaries from Zimmers.net, we can attest the character ROMs are OK ✓
We can also know for sure that it is the VIC20#1 BASIC ROM that has 2 bits wrong – is it a problem?
and finally, we can confirm that the VIC20#2 kernal fully matches kernal.901486-06.bin. ✓
So, all VIC20#2 ROMs checks OK ✓✓✓ !
At this point, we can declare a dead Kernal ROM and a doubtful BASIC ROM. — At least we have a full set to continue testing.
Dead component score:
VIC20#1 : 2 dead chips + 1 broken crystal + 1 doubtful chip
VIC20#2 : 0Next step is to check CPU.
We are All Geeks! 